Photoactive pentaerythritol derivatives and orientation layers

ABSTRACT

Alignment layers for use in liquid crystal devices comprising photoactive pentaerythritol derivatives. 1,2,3,4-trakiskis(8-[2-oxo-2H-1-benzopyran-7-yloxy]octanoyloxy)pentaerythritol is but one example.

This application is a 371 of PCT/GB00/0825 filed Mar. 7, 2000 and claimspriority to United Kingdom 99005579, filed Mar. 12, 1999.

BACKGROUND OF THE INVENTION

The present invention describes materials and methods for achievingalignment of liquid crystal materials on a substrate surface and devicesfabricated using these methods and materials.

Liquid crystal display devices (LCDs) or light shutters generallycomprise a layer of liquid crystalline material between two solidsubstrates to form a cell. These substrates are generally coated with aconducting material, such as Indium/Tin Oxide (ITO) to form electrodesor electrode patterns. An electric field applied across the cell orbetween the electrodes switches the liquid crystal between differentmolecular arrangements or states. Thus the light transmission throughthe cell can be modulated depending on the cell configuration, the typeof liquid crystalline material, the presence of polarisers, etc. Apreferred molecular alignment direction and pretilt angle (θ) isimparted by an alignment layer on top of the electrodes and in contactwith the liquid crystalline material.

It is well known in the art that fabrication of liquid crystal deviceswhich have advantageous performance and low defect densities requirescontrol of the alignment of the liquid crystalline material at thesurfaces of the device. Different types of liquid crystal alignment havebeen described. Homeotropic alignment refers to an alignment in whichthe unique optical axis of a liquid crystal phase is held perpendicularto the adjacent surface.

Planar alignment, sometimes referred to as homogeneous alignment, refersto alignment in which the unique optic axis of the liquid crystal phaselies parallel to the adjacent surface. Planar alignment may also imposea direction in which the optic axis of the liquid crystal lies in theplane of the adjacent surface.

Tilted planar alignment or tilted homogeneous alignment refer toalignment in which the liquid crystal unique optic axis lies at anangle, termed the pretilt angle (θ) from the plane of the adjacentsurface. The pretilt angle may be as small as a fraction of one degreeor as large as several tens of degrees.

Tilted homeotropic alignment refers to an alignment in which the opticaxis of the liquid crystal lies tilted away from the normal to theadjacent surface. This deviation is again termed a pretilt angle.

In liquid crystal devices, said alignment geometries are chosen and usedin combination to achieve specific optical and electro-optic propertiesfrom the device and may be combined in new ways or with new liquidcrystalline mixtures to provide new types of devices.

Several methods are known in the art by which defined liquid crystalalignment may be achieved. Deposition of a polymer layer, for example apolyimide layer, on the substrate surface followed by mechanical rubbingprovides a pretilted planar alignment. A planar alignment or tiltedplanar alignment may also be achieved by evaporating a variety ofinorganic substances, for example SiO_(x), onto the surface from anoblique angle of incidence. A disadvantage of this method is that itrequires slow and costly vacuum processing. A further disadvantage isthat the resulting evaporated layer may show a high capacity to absorbcontaminants onto itself from the environment or from other materialsused in fabrication of the device.

A homeotropic alignment can be obtained by depositing a surfactant, forexample a quatemary ammonium salt, onto the surface from solution in asuitable solvent. A disadvantage of this treatment is that theresistivity of the liquid crystal device may be lowered by thesurfactant and the resulting alignment may also show poor stability.

Structured alignment patterns of subpixel size and above can be achievedby illumination of a polymer layer containing photochemically orientabledyes or photochemically dimerisable and/or isomerisable molecules, asdescribed, for example, in EP-A-0445629. A disadvantage of this methodis that the solubility of the dye molecules in the polymer matrix islimited and the chemical and photochemical stability over time isinsufficient.

Another method for achieving structured non-contact orientation is thephotodimerisation of polymers incorporating photodimerisable groups,such as cinnamate or coumarin derivatives, as described, for example, inJpn. J. Appl. Phys., Vol., 31, 2155 (1995) and EP-A-9410699.0. Adisadvantage of these materials is the polydispersity of the materialsproduced by polymerisation. This requires, for example, differentsolution concentrations for spin coating depending on the averagemolecular weights of the polymers which can not be determined with anygreat accuracy and which are often not reproducible from one batch toanother. This can give rise to unreproducible alignment as well as alsorequiring repeated purification cycles of the polymer product in orderto remove unreacted monomer and oligomers. The attachment of low molarmass photoreactive units to monodispersed polymer backbones can lead topolymers with unreacted sites, which can give rise to dielectricbreakdown of cells containing such materials. This is especiallyimportant for active matrix devices.

An object of this invention is to provide means of achieving a definedsurface alignment of a liquid crystalline material on a substratesurface, which does not require mechanical rubbing or other methods ofphysical contact which may damage the surface or structures on thesurface. This is especially important for active matrix displays basedon the use of surface mounted thin film transistors. Static electricityor dust caused by mechanical rubbing or buffing polymer layers, such aspolyimide or polyamide, in order to induce a unidirectional alignmentdue to microgrooves can cause defects in thin film transistors and leadto dielectric breakdown. Such alignment layers also suffer from thedisadvantage that the microgrooves possess inherent defects themselves,which can result in random phase distortion and light scattering. Thisimpacts detrimentally on the optical appearance of the displays or theefficiency of the light shutters. Additionally, mechanical buffing doesnot allow locally oriented regions of the surface to be aligned withdifferent azimuthal angles. This is a substantial drawback sincesub-pixelisation can lead to higher contrast and an improved opticalefficiency.

SUMMARY OF THE INVENTION

According to this invention materials are provided of Formula I:

where

X₁₋₈ are each independently selected from: H, halogen, CN, OH, straightor branched chain alkyl having from 1 to 16 carbon atoms, where one ormore non-adjacent CH₂ groups may be substituted by CH(CN), CH(CF₃), CHF,CHCl, CH(CH₃);

S₁₋₈ are spacer units;

PG₁₋₄ are photopolymerisable/dimerisable groups

m₁, m₂, m₃, and m₄ are each independently selected from the integers 1and 0;

A₁₋₈ are each independently selected from the aromatic rings:

where ˜ indicates a sigma bond between part of the molecule shown informula I and a carbon atom at any position in one of the aromaticrings;

and where the CH groups present in the aromatic rings may each beindependently substituted by C(CN), C(CF₃), C-halogen, C(CH₃), CR, whereR is selected from straight or branched chain alkyl and may include from1 to 8 carbon atoms and including where one or more non-adjacent CH₂groups may be substituted by CH(CN), CH(CF₃), CHF, CHCl, CH(CH₃).

Preferably the spacer groups S₁₋₄ are, independently of one another,selected from groups having the general formula:

L₁—(CH₂)_(n)—L₂

where: n=1 to 30, where each CH₂ group present in the chain linking L₁and L₂ may be independently substituted by CH(CN), CH(CF₃), CHF, CHCl,CH(CH₃), L₁ and L₂ are independently selected from: single covalentbond, O, COO, OOC, CH₂O, and OCH₂. More preferably S₁₋₄ areindependently selected from oxycarbonylalkanoyloxy, oxyalkoxy,oxycarbonylalkoxy, oxyalkanoyloxy, oxycarbonylphenoxyalkanoyloxy,oxyalkoxyalkyl containing from 1-16 carbon atoms.

In a preferred embodiment spacer groups S₅₋₈ are each independentlyselected from: COO, OOC, C≡C, C═C, single covalent bond.

Preferably the photopolymerisable/dimerisable groups PG₁₋₄ are eachindependently selected from:

where a sigma bond exists between part of the molecule shown in formulaI and any one of the four C atoms that are in the benzene ring to whichG is fused and that do not form part of the ring G; and where CH groupspresent in the benzene ring to which the ring G is fused may each beindependently substituted by C(CN), C(CF₃), C-halogen, C(CH₃), CR, whereR is selected from straight or branched chain alkyl and may include from1 to 8 carbon atoms and including where one or more non-adjacent CH₂groups may be substituted by CH(CN), CH(CF₃), CHF, CHCl, CH(CH₃);

where G is independently selected from:

and where J is independently selected from:

R₁ may be H, halogen, CN, NO₂, NCS, SCN, alkyl with 1 to 12 carbon atomswhich is optionally substituted with one or more fluorines and in whichoptionally 1 or 2 non-adjacent methylene units (CH₂) can be replaced byoxygen, COO, OOC, CO and/or CH═CH;

R₂ may be H or C₁₋₁₀ alkyl;

D₁ may be H, alkyl or alkoxy with 1 to 8 carbon atoms, trifluoromethyl,or phenyl which may be substituted from one and up to and including allavailable substitution positions with one or more of the groups selectedfrom CN, halogen, NO₂;

E₁ may be H, alkyl or alkoxy with 1 to 8 carbon atoms, cyano, or COOR₅;

R₅ may be C₁₋₁₀ alkyl.

D₂ may be H, alkyl or alkoxy with 1 to 8 carbon atoms, trifluoromethyl,or phenyl which may be substituted from one and up to and including allavailable substitution positions with one or more of the groups selectedfrom CN, halogen, NO₂;

E₂ may be H, alkyl or alkoxy with 1 to 8 carbon atoms, cyano, or COOR₆;

R₆ may be C₁₋₁₀ alkyl.

D₃ may be H, alkyl or alkoxy with 1 to 8 carbon atoms, trifluoromethyl,or phenyl which may be substituted from one and up to and including allavailable substitution positions with one or more of the groups selectedfrom CN, halogen, NO₂;

E₃ may be H, alkyl or alkoxy with 1 to 8 carbon atoms, cyano, or COOR₇;

R₇ may be C₁₋₁₀ alkyl.

D₄ may be H, alkyl or alkoxy with 1 to 8 carbon atoms, trifluoromethyl,or phenyl which may be substituted from one and up to and including allavailable substitution positions with one or more of the groups selectedfrom CN, halogen, NO₂;

E₄ may be H, alkyl or alkoxy with 1 to 8 carbon atoms, cyano, or COOR₈;

R₈ may be C₁₋₁₀ alkyl.

D₅ may be H, alkyl or alkoxy with 1 to 8 carbon atoms, trifluoromethyl,or phenyl which may be substituted from one and up to and including allavailable substitution positions with one or more of the groups selectedfrom CN, halogen, NO₂;

E₅ may be H, alkyl or alkoxy with 1 to 8 carbon atoms, cyano, or COOR₉;

R₉ may be C₁₋₁₀ alkyl.

D₆ may be H, alkyl or alkoxy with 1 to 8 carbon atoms, trifluoromethyl,or phenyl which may be substituted from one and up to and including allavailable substitution positions with one or more of the groups selectedfrom CN, halogen, NO₂;

E₅ may be H, alkyl or alkoxy with 1 to 8 carbon atoms, cyano, or COOR₉;

R₉ may be C₁₋₁₀ alkyl.

Examples of the term “alkyl with 1 to 12 carbon atoms which isoptionally substituted with one or more fluorines and in whichoptionally 1 or 2 non-adjacent methylene units (CH₂) can be replaced byoxygen, COO, OOC, CO and/or CH═CH” include in the present applicationstraight-chain and branched (optionally chiral) residues such as alkyl,alkenyl, alkoxy, alkenyloxy, alkoxyalkyl, alkoxyalkenyl, 1-fluoroalkyl,terminal trifluoromethylalkyl, terminal difluoromethylalkyl, terminaltrifluoromethylalkoxy, and the like with 1 or, 2 to 16 carbon atoms.Examples of preferred residues are methyl, ethyl, propyl, butyl, pentyl,hexyl, heptyl, octyl, 1-methylpropyl, 1-methylheptyl, 2-methylbutyl,3-methyl pentyl, vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 3-butenyl,3E-pentenyl, 3E-hexenyl, 3E-hexenyl, 4-pentenyl, 4Z-hexenyl, 5-hexenyl,6-heptenyl, 7-octenyl, methoxy, ethoxy, propyloxy, butyloxy, pentyloxy,hexyloxy, octyloxy, 1-methylpropyloxy, 1-methylheptyloxy,2-methylbutyloxy, 1-fluoropropyl, 2-fluoropropyl, 2,2-difluoropropyl,3-fluoropropyl, 3,3-difluoropropyl, 3,3,3-trifluoropropyl and the like.Especially preferred residues possess 1 or, respectively, 2 to 6 carbonatoms.

The term “halogen” may represent in the present application fluorine,chlorine, bromine and iodine, but especially fluorine and chlorine.

According to an aspect of this invention a method is provided forforming an alignment layer on a surface of a liquid crystal cell wall,the method comprising the steps: depositing a layer of a materialcomprising at least one compound of Formula I on the surface; andexposing the material to actinic radiation.

Preferably the method for forming an alignment layer further comprisesthe step of controlling the exposure time and/or intensity of theactinic radiation used to provide a selected value of pretilt in aliquid crystal placed in contact with the exposed layer.

Preferably the radiation includes light, with a wavelength of 250-450nm. More preferably the radiation is light with a wavelength of 300-400nm.

According to a further aspect of this invention a liquid crystal devicecomprises a layer of a liquid crystal material contained between twocell walls both carrying electrode structures and surface treated toprovide an alignment layer for liquid crystal molecules;

characterised in that:

the alignment layer comprises a compound of Formula 1 that has beenexposed to actinic radiation.

Preferably the alignment layer comprises a compound of Formula I thathas been exposed to actinic radiation, the exposure time and/orintensity of the actinic radiation used being controlled to provide aselected value of pretilt in a liquid crystal placed in contact with theexposed layer.

Preferably the radiation includes light, with a wavelength of 250-450nm. More preferably the radiation is light with a wavelength of 300-400nm.

Compounds of Formula I can be prepared by various routes fromcommercially available starting materials. Typicallytetrakis(hydroxymethyl)methane (pentaerythritol) can be esterified withω-halogenoalkanoic acids in the presence of N,N-dicyclohexylcarbodiimideand 4-(dimethylamino)pyridine and a polar solvent, such asN,N-dimethylformamide or dichloromethane. The resultant bromides canthen be alkylated in a Williams ether synthesis with phenolsincorporating a photoisomerisable/dimerisable group, such as coumarin orcinnamate, in the presence of a base, such as potassium carbonate, and apolar solvent, such as cyclohexanone or ethyl-methylketone. The bromidescan also be esterified with a photoisomerisable/dimerisable group, suchas cinnamic acids, in the presence of DBU and a non polar solvent, suchas toluene or benzene. Similarly pentaerythritol can be alkylated withω-halogenoalkanols protected, for example as the THP derivative, in thepresence of base, such as potassium tert.-butylate, and a polar solvent,such as 1,2-dimethoxyethane or ethylene glycol dimethyl ether. Afterdeprotection the resultant alcohols can then be esterified with aphotoisomerisable/dimerisable group, such as aromatic acidsincorporating a cinnamate or coumarin moiety, in the presence ofN,N-dicyclohexylcarbodiimide and 4-(dimethylamino)pyridine and a polarsolvent, such as N,N-dimethylformamide or dichloromethane. The alcoholscan also be alkylated in a Mitsunobu reaction, with aphotoisomerisable/dimerisable group, such as 6-hydroxycoumarin,7-hydroxycoumarin (umbelliferone) or alkyl hydroxycinnamates, in thepresence of a dehydrating agent, such as diethyl azodicarboxylate andtriphenyl phosphine, and a polar solvent, such as tetrahydrofuran orN,N-dimethylformamide.

The photocross-linkable groups, such as cinnamic acids, cinnamateesters, cinnamonitriles, styrenes, stilbenes, vinylnaphthalenes,vinylpyridines, maleimides, thymines, coumarins, are generally areeither commercially available or readily accessible, for examplecoumarin and cinnamate derivatives can be prepared according toliterature methods, such as the Perkin, Pechmann, Knoevenagel,Wittig-Homer, Heck or sigmatropic rearrangement reactions (OrganicReactions, 1, 210, 1942; Organic Reactions, 15, 204, 1967; Synthesis,131, 1978; J. Mol. Cat., 88, L113, 1994; J. Chem. Soc. Perkin Trans. I,1753, 1987).

In order to obtain alignment layers in regions selectively limited byarea, a solution of the photoactive pentraerythritol derivative can, forexample, firstly be prepared and then spread out using a spin-coatingapparatus on a carrier coated with an electrode , e.g., a glass platecoated with indium-tin oxide (ITO) such that homogeneous layers of0.05-50 μm thickness result. Subsequently or simultaneously, irradiationcan be applied to the region to be isomerised and/or dimerised(cross-linked), e.g., with a mercury high pressure lamp, a xenon lamp ora UV laser utilising a polariser and optionally a mask for the formationof structures. The duration and irradiation depends on the capacity ofthe individual lamps and can vary from a few minutes to several hours.The cross-linking can, however, also effected by irradiating thehomogeneous layer using filters which, e.g., let through only radiationsuitable for the cross-linking reaction. Photosensitisors, such asacetophenone or benzophenone may be added to shorten the illuminationtime required for cross-linking. Non-zero tilt angles (θ) may be inducedby illumination with plane polarised light from a non-perpendicularangle to the plane of the substrate.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described, by way of example only, withreference to the following examples and diagrams:

FIG. 1 is a plan view of a matrix multiplex addressed liquid crystaldisplay;

FIG. 2 is a cross-section of a display such as FIG. 1 used in atransmissive mode;

FIG. 3 is similar to FIG. 2 but operates in a reflective mode; and

FIG. 4 is a schematic representation of the apparatus used to illuminatethe photocross-linkable propane derivatives on a suitable substrate tobe used as part of a liquid crystal device.

DETAILED DESCRIPTION OF INVENTION

The device of FIGS. 1, 2 and 3 comprises a liquid crystal cell 1 formedby a layer of a liquid crystal material 2 contained between two glasswalls 3, 4 spaced typically 1 to 15 μm apart by a spacer ring 5. Theinside faces of both walls 3, 4 are coated with electrodes 6. Theelectrodes may be of sheet like form covering the complete wall, orformed into for example, strip electrodes to provide an array ofaddressable electrode intersections. The walls are also coated with analigning layer (not shown) of material described by the currentinvention.

If the material 2 is nematic then the device may be the known supertwisted nematic device, also known as a STN device. In this casepolarisers 13 are used to distinguish between the device voltage ON andOFF states.

The liquid crystal material may be nematic, chiral nematic(cholesteric), or smectic (e.g., ferroelectric) material. The device maybe used as a display device, e.g., displaying alpha numeric information,or an x, y matrix displaying information. alternatively the device mayoperate as a shutter to modulate light transmission, e.g., as a spatiallight modulator, or as a privacy window.

For passive matrix devices (shown in FIG. 1) strip like row electrodes 6₁, to 6 _(m), e.g. of InSnO₂ are formed on one wall 3 and similar columnelectrodes 7 ₁ to 7 _(n) formed on the other wall 4. With m-rowelectrodes and n-column electrodes this forms an mxn matrix ofaddressable elements. Each element is formed by the interaction of a rowand column electrode. For active matrix devices a discrete nonlineardevice eg a transistor or diode is associated with each pixel.

For the passive matrix device a row driver supplies voltage to each rowelectrode 6. Similarly a column drive 9 supplies voltages to each columnelectrode 7. Control of applied voltages is from a control logic 10which receives power from a voltage source 11 and timing from a clock12.

For an active device e.g., a thin film transistor active matrix liquidcrystal device (TFT AMLCD) three types of electrodes are present, pixel,scanning and signal electrodes as well as a common electrode on theopposite side of the liquid crystal. The control electrode operates thegate such that the voltage on the signal electrode is applied to therelevant pixel electrode.

An example of the use of a material and device embodying the presentinvention will now be described with reference to FIG. 2.

The liquid crystal device consists of two transparent plates, 3 and 4,for example made from glass, in the case of an active matrix devicethese will usually be ofaluminosilicate (alkali free) glass often with apassivation layer of SiO₂. For an active matrix display the activedevices eg thin film transistors, are fabricated and the colour filterlayer is added for a full colour display. These plates are coated ontheir internal face with transparent conducting electrodes 6 and 7,often ITO which is patterned using photolithography techniques. Thetransparent plates 3 and 4 are coated with a photoactive sample,comprising one or more compounds according to the invention. A typicalcoating procedure involves the dissolution of one of the compounds ofthe invention in a solvent, for example cyclopentanone, followed by spincoating of the photoactive compound on the transparent plate. Once thephotoactive compound has been coated onto the plates it is exposed toactinic radiation to induce cross-linking of the photoactive molecules.The cross-linking process can be monitored by measuring thebirefringence of the alignment layer. The intersections between eachcolumn and row electrode form an x, y matrix of addressable elements orpixels. A spacer 5 e.g. of polymethyl methacrylate separates the glassplates 3 and 4 to a suitable distance e.g. 2-7 microns preferably 4-6microns. Liquid crystal material 2 is introduced between glass plates3,4 by filling the space in between them. This may be done by flowfilling the cell using standard techniques. The spacer 5 is sealed withan adhesive in a vacuum using an existing technique. Polarisers 13 maybe arranged in front of and behind the cell.

The device may operate in a transmissive or reflective mode. In theformer, light passing through the device, e.g. from a tungsten bulb, isselectively transmitted or blocked to form the desired display. In thereflective mode a mirror, or diffuse reflector, (16) is placed behindthe second polariser 13 to reflect ambient light back through the celland two polarisers. By making the mirror partly reflecting the devicemay be operated both in a transmissive and reflective mode.

The alignment layers have two functions, one to align contacting liquidcrystal molecules in a preferred direction and the other to give a tiltto these molecules—a so called surface tilt—of a few degrees typicallyaround 4° or 5°. In an alternative embodiment a single polariser and dyematerial may be combined. The materials of the current invention mayalso be used in LCDs with an actively addressed matrix e.g. thin filmtransistors (TFT-LCDs) or a passively addressed matrix e.g., dual scanSTN.

The apparatus of FIG. 4 used to generate photoinduced anisotropy in aphotoactive sample 17, comprising a photoactive pentaerythrol derivativeaccording to the invention, using linearly polarised UV light comprisesa radiation source e.g. an argon ion laser 18 (Spectra Physics, Model2045). The laser beam operating at 300.5 nm has a polarisation direction(E). The laser beam was expanded by using a quartz lens beam expander19. The film anisotropy is measured by determining the inducedbirefringence against UV exposure time using a He—Ne laser 20 (632.8 nm)modulated by a rotating chopper 21. The probing wavelength does notperturb the anisotropy inducing process since it is far away from anyintrinsic absorption band in either the starting material orphotoproduct. During the UV exposure of the sample, the photoinducedbirefringence was monitored using two crossed polarisers, P1 and P2arranged at +/−45° with respect to the vertical polarisation of the UVlaser beam. The intensity of the beam from the probing He—Ne laser 20,which then passed through the sample, was detected by a photodetector22. The-birefringence measuring signal taken from the photodetector 22was processed by a phase sensitive lock-in amplifier 20 and recorded bya computer 24. Data were plotted as birefringence versus UV exposuretime. In this way the cross-linking process can be induced andmonitored.

The photocross-linkable pentaerythritol derivatives and resultantalignment layers may be produced as described, by way of example only,in the following examples; K signifies the crystalline state, Isignifies the isotropic phase, T_(g) is the glass transitiontemperature.

EXAMPLE 1 Preparation of1,2,3,4-Tetrakiskis(8-[2-oxo-2H-1-benzopyran-7-yloxy]octanoyloxy)pentaerythritol

A mixture of 3.0 g 1,2,3,4-tetrakis(8-bromooctanoyloxy)pentaerythritol,2.0 g 7-hydroxycoumarin, 5.0 g anhydrous potassium carbonate and 50 mlethyl-methyl ketone was heated under gentle reflux overnight. Thereaction mixture was filtered to remove inorganic material and thefiltrate evaporated down. The residue was purified by columnchromatography on silica gel using hexane/ethyl acetate (1/1 v/v) aseluent and recrystallisation from acetonitrile to yield 2.1 g of1,2,3,4-tetrakis(8-[2-oxo-2H-1-benzopyran-7-yloxy]octanoyloxy)pentaerythritolas an oil.

The 1,2,3,4-tetrakis(8-bromooctanoyloxy)pentaerythritol required asstarting material was prepared as follows:

12.0 g Dicyclohexylcarbodiimide was added to a solution of 2.0 gpentaerythritol, 13.0 g 8-bromooctanoic acid, 1.8 g4-(dimethylamino)pyridine and 50 ml dichloromethane at 0° C. Thereaction mixture was stirred overnight at room temperature, filtered toremove inorganic material and the filtrate evaporated down. The residuewas purified by column chromatography on silica gel using hexane/ethylacetate (1/1 v/v) as eluent and recrystallisation from acetonitrile toyield 6.5 g of 1,2,3,4-tetrakis(8-bromooctanoyloxy)pentaerythritol as anoil.

The following compounds can be prepared in an analogous manner:

1,2,3,4-tetrakis(3-[2-oxo-2H-1-benzopyran-7-yloxy]propanoyloxy)pentaerythritol.

1,2,3,4-tetrakis(4-[2-oxo-2H-1-benzopyran-7-yloxy]butanoyloxy)pentaerythritol.

1,2,3,4-tetrakis(5-[2-oxo-2H-1-benzopyran-7-yloxy]pentanoyloxy)pentaerythritol.

1,2,3,4-tetrakis(6-[2-oxo-2H-1-benzopyran-7-yloxy]hexanoyloxy)pentaerythritol.

1,2,3,4-tetrakis(7-[2-oxo-2H-1-benzopyran-7-yloxy]heptanoyloxy)pentaerythritol.

1,2,3,4-tetrakis(9-[2-oxo-2H-1-benzopyran-7-yloxy]nonanoyloxy)pentaerythritol.

1,2,3,4-tetrakis(10-[2-oxo-2H-1-benzopyran-7-yloxy]decanoyloxy)pentaerythritol.

1,2,3,4-tetrakis(3-[2-oxo-2H-1-benzopyran-6-yloxy]propanoyloxy)pentaerythritol.

1,2,3,4-tetrakis(4-[2-oxo-2H-1-benzopyran-6-yloxy]butanoyloxy)pentaerythritol.

1,2,3,4-tetrakis(5-[2-oxo-2H-1-benzopyran-6-yloxy]pentanoyloxy)pentaerythritol.

1,2,3,4-tetrakis(6-[2-oxo-2H-1-benzopyran-6-yloxy]hexanoyloxy)pentaerythritol.

1,2,3,4-tetrakis(7-[2-oxo-2H-1-benzopyran-6-yloxy]heptanoyloxy)pentaerythritol.

1,2,3,4-tetrakis(8-[2-oxo-2H-1-benzopyran-6-yloxy]octanoyloxy)pentaerythritol.

1,2,3,4-tetrakis(9-[2-oxo-2H-1-benzopyran-6-yloxy]nonanoyloxy)pentaerythritol.

1,2,3,4-tetrakis(10-[2-oxo-2H-1-benzopyran-6-yloxy]decanoyloxy)pentaerythritol.

1,2,3,4-tetrakis[3-(4-[(E)-methoxycarbonylethenyl]phenoxy)propanoyloxy]pentaerythritol.

1,2,3,4-tetrakis[3-(4-[(E)-ethoxycarbonylethenyl]phenoxy)propanoyloxy]pentaerythritol.

1,2,3,4-tetrakis[4-(4-[(E)-ethoxycarbonylethenyl]phenoxy)butanoyloxy]pentaerythritol.

1,2,3,4-tetrakis[5-(4-[(E)-ethoxycarbonylethenyl]phenoxy)pentanoyloxy]pentaerythritol.

1,2,3,4-tetrakis[6-(4-[(E)-ethoxycarbonylethenyl]phenoxy)hexanoyloxy]pentaerythritol.

1,2,3,4-tetrakis[7-(4-[(E)-ethoxycarbonylethenyl]phenoxy)heptanoyloxy]pentaerythritol.

1,2,3,4-tetrakis[8-(4-[(E)-ethoxycarbonylethenyl]phenoxy)octanoyloxy]pentaerythritol.

1,2,3,4-tetrakis[8-(4-{2-[(E)-ethoxycarbonylethenyl]thiophen-5-yl}phenoxy)octanoyloxy]pentaerythritol.

1,2,3,4-tetrakis[8-(4-{2-[(E)-ethoxycarbonylethenyl]furan-5-yl}phenoxy)octanoyloxy]pentaerythritol.

1,2,3,4-tetrakis[8-(4-{2-[(E)-ethoxycarbonylethenyl]pridin-5-yl}phenoxy)octanoyloxy]pentaerythritol.

1,2,3,4-tetrakis[8-(4-{2-[(E)-ethoxycarbonylethenyl]primidin-5-yl}phenoxy)octanoyloxy]pentaerythritol.

1,2,3,4-tetrakis[3-(4-[(E)-ethoxycarbonylethenyl]biphenyl-4′-yloxy)propanoyloxy]pentaerythritol.

1,2,3,4-tetrakis[4-(4-[(E)-ethoxycarbonylethenyl]biphenyl-4′-yloxy)butanoyloxy]pentaerythritol.

1,2,3,4-tetrakis[5-(4-[(E)-ethoxycarbonylethenyl]biphenyl-4′-yloxy)pentanoyloxy]pentaerythritol.

1,2,3,4-tetrakis[6-(4-[(E)-ethoxycarbonylethenyl]biphenyl-4′-yloxy)hexanoyloxy]pentaerythritol,Mpt 187° C.

1,2,3,4-tetrakis[7-(4-[(E)-ethoxycarbonylethenyl]biphenyl-4′-yloxy)heptanoyloxy]pentaerythritol.

1,2,3,4-tetrakis[8-(4-[(E)-ethoxycarbonylethenyl]biphenyl-4′-yloxy)octanoyloxy]pentaerythritol.

EXAMPLE 2 Preparation of1,2,3,4-Tetrakis(4-[(E)-ethoxycarboxyethenyl]biphenyl-4′-yloxy)pentaerythritol

2.6 g Diethyl azodicarboxylate was added to a solution of 3.7 g4-[(E)-ethoxycarboxyethenyl]4′-hydroxybiphenyl, 0.57 g pentaerythritol,3.9 g triphenyl phosphine and 50 ml tetrahydrofuran at 0° C. Thereaction mixture was stirred overnight at room temperature and thenevaporated down with silica gel. The resultant powder was purified bycolumn chromatography on silica gel using hexane/ethyl acetate (1/1 v/v)as eluent and recrystallisation from ethanol to yield 2.1 g of1,2,3,4-tetrakis(4-[(E)-ethoxycarboxyethenyl]biphenyl-4′-yloxy)pentaerythritol;Mpt 241° C.

The 4-[(E)-ethoxycarboxyethenyl]-4′-hydroxybiphenyl required as startingmaterial is prepared as follows:

A mixture of 10.0 g 4-bromo-4′-hydroxybiphenyl, 6.9 g methyl acrylate,8.1 g triethylamine, 0.1 g palladium(II)acetate, 0.4 gtri(o-toluyl)phosphine and 30 ml acetonitrile was heated under gentlereflux overnight. The reaction mixture was diluted with 200 mlacetonitrile, filtered to remove inorganic material and the filtrateevaporated down. The residue was purified by precipitation from adichloromethane solution into hexane and recrystallisation fromacetonitrile to yield 7.2 g of4-[(E)-ethoxycarboxyethenyl]-4′-hydroxybiphenyl; Mpt 228° C.

The following compounds can be prepared in an analogous manner

1,2,3,4-tetrakis(2-oxo-2H-1-benzopyran-7-yloxy)pentaerythritol.carbonyl)phenoxy]dodecanoyloxy)propane.

1,2,3,4-tetrakis(2-oxo-2H-1-benzopyran-6-yloxy)pentaerythritol.carbonyl)phenoxy]dodecanoyloxy)propane.

1,2,3,4-tetrakis(4-[(E)-methoxycarboxyethenyl]phenoxy)pentaerythritol.

1,2,3,4-tetrakis(4-[(E)-ethoxycarboxyethenyl]phenoxy)pentaerythritol.

1,2,3,4-tetrakis(4-[(E)-methoxycarboxyethenyl]biphenyl-4′-yloxy)pentaerythritol.

1,2,3,4-tetrakis(4-[(E)-methoxycarboxyethenyl]-p-terphenyl-4″-yloxy)pentaerythritol.

EXAMPLE 3 Preparation of1,2,3,4-Tetrakiskis(6-[4-{2-oxo-2H-1-benzopyran-7-yloxycarbonyl}phenoxy]hexanoyloxy)pentaerythritol

A mixture of 0.7 g 1,2,3,4-tetrakis(5-bromohexanoyloxy)pentaerythritol,1.0 g 2-oxo-2H-1-benzopyran-7-yl 4-hydroxybenzoate, 5.0 g anhydrouspotassium carbonate and 50 ml ethyl-methyl ketone is heated under gentlereflux overnight. The reaction mixture is filtered to remove inorganicmaterial and the filtrate evaporated down. The residue is purified bycolumn chromatography on silica gel using hexane/ethyl acetate (1/1 v/v)as eluent and recrystallisation from acetonitrile to yield 1.2 g of1,2,3,4-tetrakiskis(6-[4-{2-oxo-2H-1-benzopyran-7-yloxycarbonyl}phenoxy]hexanoyloxy)pentaerythritol.

The 2-oxo-2H-1-benzopyran-7-yl 4-hydroxybenzoate required as startingmaterial was prepared as follows:

13.4 g triphenylphosphine is added to a solution of 8.3 g7-hydroxycoumarin, 10.0 g 4-methoxycarbonyloxybenzoic acid, 8.9 gdiethyl azodicarboxylate and 50 ml tetrahydrofuran at 0° C. The reactionmixture was stirred overnight at room temperature, filtered to removeinorganic material and the filtrate evaporated down, taken up in warmhexane, filtered to remove inorganic material and evaporated down again.The residue is purified by column chromatography on silica gel usinghexane/ethyl acetate (1/1 v/v) as eluent and recrystallisation fromacetonitrile to yield 12.2 g of 2-oxo-2H-1-benzopyran-7-yl4-methoxycarbonyloxybenzoate.

50 ml of saturated ethanolic ammonia solution is added dropwise to asolution of 12.2 g of 2-oxo-2H-1-benzopyran-7-yl4-methoxycarbonyloxybenzoate in 200 ml of ethanol. The reaction mixtureis stirred for two hours, evaporated down and taken up in 100 ml diethylether. This solution is washed with water, dried, filtered to removeinorganic material and the filtrate evaporated down. The residue ispurified by column chromatography on silica gel using hexane/ethylacetate (1/1 v/v) as eluent and recrystallisation from acetonitrile toyield 9.5 g of 2-oxo-2H-1-benzopyran-7-yl 4-hydroxybenzoate.

The following compounds can be prepared in an analogous manner:

1,2,3,4-tetrakiskis(3-[4-{2-oxo-2H-1-benzopyran-7-yloxycarbonyl}phenoxy]propanoyloxy)pentaerythritol.

1,2,3,4-tetrakiskis(4-[4-{2-oxo-2H-1-benzopyran-7-yloxycarbonyl}phenoxy]butanoyloxy)pentaerythritol.

1,2,3,4-tetrakiskis(5-[4-{2-oxo-2H-1-benzopyran-7-yloxycarbonyl}phenoxy]pentanoyloxy)pentaerythritol.

1,2,3,4-tetrakiskis(7-[4-{2-oxo-2H-1-benzopyran-7-yloxycarbonyl}phenoxy]heptanoyloxy)pentaerythritol

1,2,3,4-tetrakiskis(8-[4-{2-oxo-2H-1-benzopyran-7-yloxycarbonyl}phenoxy]octanoyloxy)pentaerythritol

1,2,3,4-tetrakiskis(3-[4-{2-oxo-2H-1-benzopyran-6-yloxycarbonyl}phenoxy]propanoyloxy)pentaerythritol.

1,2,3,4-tetrakiskis(4-[4-{2-oxo-2H-1-benzopyran-6-yloxycarbonyl}phenoxy]butanoyloxy)pentaerythritol.

1,2,3,4-tetrakiskis(5-[4-{2-oxo-2H-1-benzopyran-6-yloxycarbonyl}phenoxy]pentanoyloxy)pentaerythritol.

1,2,3,4-tetrakiskis(6-[4-{2-oxo-2H-1-benzopyran-6-yloxycarbonyl}phenoxy]hexanoyloxy)pentaerythritol.

1,2,3,4-tetrakiskis(7-[4-{2-oxo-2H-1-benzopyran-6-yloxycarbonyl}phenoxy]heptanoyloxy)pentaerythritol

1,2,3,4-tetrakiskis(8-[4-{2-oxo-2H-1-benzopyran-6-yloxycarbonyl}phenoxy]octanoyloxy)pentaerythritol

EXAMPLE 4 Preparation of Aligned Twisted and Planar Nematic Cells

A 2 w/w % solution of1,2,3,4-tetrakis(8-[2-oxo-2H-1-benzopyran-7-yloxy]octanoyloxy)pentaerythritolin cyclopentanone was spin coated at 3000 rpm for 30 seconds onto indiumtin oxide glass substrates (24×25 mm²). The coated substrates were driedat 80° C. for 30 min and then illuminated with linearly polarisedultra-violet light at 300.5 nm from an argon ion laser using the set-upshown in FIG. 4. The film anisotropy against UV exposure time wasmeasured by determining the induced birefringence using a He-Ne laser at632.8 nm. By the method described Twisted and non-twisted nematic cellswere prepared by combining the photoaligned substrate with aunidirectionally rubbed polyimide substrate whose orientation directionwas either parallel or orthogonal to that of the photoaligned substrate.An uniform cell gap was obtained by using mylar spacers (17 μm). Thecells were filled with a nematic mixture (Merck E202) at 89° C. undervacuum by capillary action. On cooling either a twisted nematic or anon-twisted cell was observed depending on how the substrates werecombined. A twist angle of 90° was found for a twisted nematic cellusing this cross-linked material exposed for 8 min at 2 mWcm⁻².

What is claimed is:
 1. A compound of Formula I:

wherein X₁₋₈ are each independently selected from: H, halogen, CN, OH,straight or branched chain alkyl having from 1 to 16 carbon atoms, whereone or more non-adjacent CH₂ groups may be substituted by CH(CN),CH(CF₃), CHF, CHCl, CH(CH₃); S₁₋₈ are spacer units; PG₁₋₄ arephotopolymerisable/dimerisable groups; m₁, m₂, m₃, and m₄ are eachindependently selected from the integers 1 and 0; A₁₋₈ are eachindependently selected from the aromatic rings:

where ˜ indicates a sigma bond between part of the molecule shown informula I and a carbon atom at any position in one of the aromaticrings; and where the CH groups present in the aromatic rings may each beindependently substituted by C(CN), C(CF₃), C-halogen, C(CH₃), CR, whereR is selected from straight or branched chain alkyl and may include from1 to 8 carbon atoms and including where one or more non-adjacent CH₂groups may be substituted by CH(CN), CH(CF₃), CHF, CHCl, CH(CH₃).
 2. Acompound according to claim 1 wherein S₁₋₄ are, independently of oneanother, selected from groups having the general formula:L₁—(CH₂)_(n)—L₂ where: n=1 to 30, where each CH₂ group present in thechain linking L₁ and L₂ may be independently substituted by CH(CN),CH(CF₃), CHF, CHCl, CH(CH₃), L₁ and L₂ are independently selected from:single covalent bond, O, COO, OOC, CH₂O, and OCH₂; and S₅₋₈ are eachindependently selected from: COO, OOC, C≡C, C═C, single covalent bond.3. A compound according to claim 1 wherein S₁₋₄ are independentlyselected from oxycarbonylalkanoyloxy, oxyalkoxy, oxycarbonylalkoxy,oxyalkanoyloxy, oxycarbonylphenoxyalkanoyloxy, oxyalkoxyalkyl containingfrom 1-16 carbon atoms.
 4. A compound according to claim 1 wherein PG₁₋₄are, independently of one another, selected from:

where a sigma bond exists between part of the molecule shown in FormulaI and any one of the four C atoms that are in the benzene ring to whichG is fused and that do not form part of the ring G, and where CH groupspresent in the benzene ring, to which the ring G is fused, may each beindependently substituted by C(CN), C(CF₃), C-halogen, C(CH₃), CR, whereR is selected from straight or branched chain alkyl and may include from1 to 8 carbon atoms and including where one or more non-adjacent CH₂groups may be substituted by CH(CN), CH(CF₃), CHF, CHCl, CH(CH₃); R₁ maybe H, halogen, CN, NO₂, NCS, SCN, alkyl with 1 to 12 carbon atoms whichis optionally substituted with one or more fluorines and in whichoptionally 1 or 2 non-adjacent methylene units (CH₂) can be replaced byoxygen, COO, OOC, CO and/or CH═CH; D, may be H, alkyl or alkoxy with 1to 8 carbon atoms, trifluoromethyl, or phenyl which may be substitutedfrom one and up to and including all available substitution positionswith one or more of the groups selected from CN, halogen, NO₂; E₁ may beH, alkyl or alkoxy with 1 to 8 carbon atoms, cyano, or COOR₅; R₅ may beC₁₋₁₀ alkyl; where G is independently selected from:

R₂ may be H or C₁₋₁₀ alkyl; D₂ may be H, alkyl or alkoxy with 1 to 8carbon atoms, trifluoromethyl, or phenyl which may be substituted fromone and up to and including all available substitution positions withone or more of the groups selected from CN, halogen, NO₂: E₂ may be H,alkyl or alkoxy with 1 to 8 carbon atoms, cyano, or COOR₆; R₆ may beC₁₋₁₀ alkyl; D₃ may be H, alkyl or alkoxy with 1 to 8 carbon atoms,trifluoromethyl, or phenyl which may be substituted from one and up toand including all available substitution positions with one or more ofthe groups selected from CN, halogen, NO₂; E₃ may be H, alkyl or alkoxywith 1 to 8 carbon atoms, cyano, or COOR₇; R₇ may be C₁₋₁₀ alkyl. D₄ maybe H, alkyl or alkoxy with 1 to 8 carbon atoms, trifluoromethyl, orphenyl which may be substituted from one and up to and including allavailable substitution positions with one or more of the groups selectedfrom CN, halogen, NO₂; E₄ may be H, alkyl or alkoxy with 1 to 8 carbonatoms, cyano, or COOR₈; R₈ may be C₁₋₁₀ alkyl. and where J isindependently selected from:

D₅ may be H, alkyl or alkoxy with 1 to 8 carbon atoms, trifluoromethyl,or phenyl which may be substituted from one and up to and including allavailable substitution positions with one or more of the groups selectedfrom CN, halogen, NO₂; E₅ may be H, alkyl or alkoxy with 1 to 8 carbonatoms, cyano, or COOR₉; R₉ may be C₁₋₁₀ alkyl; D₆ may be H, alkyl oralkoxy with 1 to 8 carbon atoms, trifluoromethyl, or phenyl which may besubstituted from one and up to and including all available substitutionpositions with one or more of the groups selected from CN, halogen, NO₂;E₆ may be H, alkyl or alkoxy with 1 to 8 carbon atoms, cyano, or COOR₁₀,R₁₀ may be C₁₋₁₀ alkyl.
 5. A method of providing an alignment layer on asurface of a liquid crystal cell wall comprising the steps of depositinga layer of material comprising at least one compound according to any ofclaims 1-4 on the surface; exposing the material to actinic radiation.6. A method according to claim 5 characterised in that the methodfurther comprises the step of controlling the exposure time and/orintensity of the actinic radiation used to provide a selected value ofpretilt in a liquid crystal placed in contact with the exposed layer. 7.A liquid crystal device comprising a layer of liquid crystal materialbetween two cell walls, the cell walls at least one carrying electrodestructures and surface treated to provide an alignment layer for liquidcrystal molecules; characterised in that the alignment layer comprises acompound according to any of claims 1-4 which has been exposed toactinic radiation.
 8. A liquid crystal device according to claim 7characterised in the alignment layer comprises a compound according toclaims 1-4 that has been exposed to actinic radiation, the exposure timeand/or intensity of the actinic radiation used being controlled toprovide a selected value of pretilt in a liquid crystal placed incontact with the exposed layer.
 9. A liquid crystal device according toclaim 6 wherein the device is an Active Matrix Device.
 10. A liquidcrystal device according to either of claims 6 and 7 wherein the deviceis an STN device.
 11. A method according to claim 5 wherein theradiation is in the range 250-450 nm.
 12. A device according to claim 6wherein the radiation is in the range 250-450 nm.